1. Field of the Invention
This invention relates to engines valves and, more particularly, the art of making titanium engine valves.
2. Description of the Prior Art
An automobile engine valve represents a challenging application for titanium based materials (the latter is used herein to mean titanium or titanium alloys in which titanium is present in an amount of at least 70% by weight). The operating temperature range of an automobile exhaust valve (head zone) exceeds that at which most titanium alloys are used in aerospace applications.
Optimally, the material properties for an automobile engine valve should vary depending upon the location within the valve. The head of the valve is subjected to a long-term, high temperature environment (up to 1400.degree. F.) which, without adequate creep resistance of the material, leads to deformation of the head over a long period of time at such high temperatures. The stem and fillet zones of such valve are subjected to lower temperatures (up to 1200.degree. F.), being more remote from the high temperature combustion chamber, but are subjected to high tensile shock and fatiguing forces from the camshaft and valve spring action, which function upon the stem and fillet of such valve. Accordingly, tensile, shock and fatigue strength are important physical characteristics in such location.
The prior art has attempted to recognize such different physical needs of an engine valve. To meet such needs, one approach has been to make the head and the stem separate items and lock them together by a variety of modes (see U.S. Pat. Nos. 2,002,641; 1,547,125; 1,230,140). Another approach has been to influence the initial casting of the valve material through local chilling or through controlled casting conditions and then to vary the microstructure by aging some localized portion of the valve after it has been forged to final shape (see U.S. Pat. Nos. 1,347,542 and 3,536,053). Unfortunately, these approaches work only with ferrous alloys. Casting of titanium leads to inferior physical properties and thus influencing the casting of titanium to achieve a dual microstructure would not be rewarding.
Because titanium offers such dramatic decrease in weight, engine valves, particularly for racing engines, have been developed from titanium alloys. Certain physical properties are potentially limiting with titanium alloys: creep strength (which is time dependent deformation at high temperatures) is limiting in the valve head area, while tensile, shock and fatigue strengths are limiting physical characteristics in the valve stem area. Although titanium alloys can be thermomechanically processed in a variety of ways to give different microstructures, dual microstructures have not been obtained nor attempted. Some alloys are processed to be very creep resistant but have only moderate strength, while other alloys have excellent strength and fatigue properties but only moderate creep resistance. All of such approaches with titanium alloy valves have produced a singular, continuous microstructure throughout the valve and have lacked the ability to obtain dual characteristics of high creep strength on the one hand and high fatigue, tensile and shock strengths on the other hand.
An object of this method invention is to provide a titanium based engine valve that has increased creep strength at high temperatures in one zone and increased tensile strength, and fatigue and shock resistance in another zone.
Another object of this invention is to fabricate a mixed alpha/beta phase titanium based engine valve and impart fine, equiaxed, microstructural grains to one zone of the valve and a colony type microstructure to another zone.